Node and a Method for use in a Wireless Communications System

ABSTRACT

The invention relates to a relay or repeater node ( 21 ) for use in a wireless communications system said node comprising a receive antenna ( 23 ) for receiving a signal through a wireless connection, an amplifier ( 30 ) for amplifying the signal and a transmit antenna ( 27 ) for forwarding the amplified signal, said node further comprising a mode switching unit ( 31 ) for switching between at least a first and a second mode of operation of the node in dependence of an amplification gain requirement. This enables optimization of the node for varying conditions in the network.

TECHNICAL FIELD

The present invention relates to a relay or repeater node for use in acellular communications system and a method of operating such a node.

BACKGROUND AND RELATED ART

As more and more communication is performed in wireless/cellularcommunication systems, the requirements on such systems are constantlyincreasing. Such requirements relate both to increased coverage andsupport of higher data rates, or preferably a combination of both.

To enhance coverage and data rate a forwarding node, such as a repeateror a relay, is often used. Such forwarding nodes typically receive thesignal and amplify it before transmitting it to the receiver. Thebenefit of using repeaters and relays stems primarily from thatsplitting a long distance in two hops allows increased data rate on eachlink as well as End-To-End total data rate.

The distinction between the repeaters and relays is not entirely clearcut. Often, the term repeater is used to denote a simple, low processingnode that receives a signal, amplifies it and sends it out. However,some repeaters also demodulate and remodulate the received signal, andeven include power control. A relay is normally considered a bit moreadvanced and complex. It typically receives a signals demodulated and(FEC)-decode it prior deciding which resource to send it onto, e.g.through scheduling. To complicate things a bit, relaying is often usedas an umbrella term, including any node forwarding any piece ofinformation.

There are many different types of repeaters, such as frequencytranslating and on-frequency repeaters. The so called on-frequencyrepeater transmits the signal on the same resource as it was receivedon. That is, it uses the same frequency and/or coding scheme andtransmits the signal immediately upon receiving it. Such a repeateravoids the resulting throughput loss, often referred to as a duplexloss, that arises in many schemes where the node forwarding data orsignal can not receive and transmit at the same time and on the samefrequency.

On the other hand, an on-frequency repeater is prone toself-interference since the same signal is received and transmitted atvirtually the same time. Therefore, the gain that can be applied in sucha repeater is limited to a level that will not cause excessiveself-interference. To minimize self-interference, a high degree ofisolation between the input antenna and the output antenna is usuallyaimed for. Further techniques for self-interference cancellation areoften applied. The latter means that the repeater internally cancels therepeater output signal that is fed back to the repeater. Even with suchmeasures to reduce self-interference, the maximum gain is limited.

The on-frequency repeater is an attractive solution to come to termswith the duplex loss seen for relays and frequency translatingrepeaters, but is often inadequate when the repeater receives a weaksignal and needs to amplify the signal and communicate to a distantreceiver to which the path loss is high.

On the other hand, repeaters that receive and transmit on differentresources, such as frequency translating repeaters avoidself-interference at the cost of introducing a duplex loss.

Hence, when designing networks a trade-off is made between the need forhigh amplification gain and optimum throughput.

A network architecture using relay nodes is disclosed in PatentApplication Publication No. US2007/0160014 A1. Patent ApplicationPublication No. U.S. 2005/0232223 discusses the use of on-frequency orfrequency-shifting repeaters.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a relay and/or repeater nodewith improved performance under varying conditions.

This object is achieved according to the present invention by a node foruse in a wireless communications system said node comprising a receiveantenna side having at least one receive antenna for receiving a signalthrough a wireless connection, an amplifier for amplifying the signaland a transmit antenna side having at least one transmit antenna forforwarding the amplified signal, said node further comprising a modeswitching unit for switching between at least a first and a second modeof operation of the node in dependence of an amplification gainrequirement.

The object is also achieved by a method of repeating a signal in acellular network, comprising the steps of

-   -   receiving the signal in a node comprising a receive antenna side        having at lest one receive antenna, a transmit antenna side        having at least one transmit antenna, and an amplifier between        said receive antenna and said transmit antenna,    -   amplifying the signal and    -   transmitting the amplified signal, characterized by the step of    -   selecting one of a first mode and a second mode of operation for        the node in dependence of an amplification gain requirement.

By enabling adaptive switching between on-frequency operation andreceive and transmitting on orthogonal resources for the relay/repeaternode the invention ensures that the repeater function can be performedwithout duplex loss whenever possible, that is, when an on-frequencyrepeating is feasible. At the same time, higher gain can be achievedwhen this is desirable, to enable supporting repeater node to receivernode links with a higher path loss, or Signal to Interference Ratio(SIR). In addition, switching to the frequency translating mode canenable improving the reception performance of the relay/repeater nodethrough the enhanced signal to interference ratio as self-interferencecan not be cancelled perfectly in the on-frequency mode.

Hence, a main benefit of this adaptive hybrid operation betweenon-frequency repeater operation and either of frequency translatingrepeater or a relay is that the spectral efficiency offered byon-frequency repeater can be used, whereas still operating in situationswhere high gain operation is occasionally required.

Another benefit is that when the relay/repeater node may experience alow SNR at receiving, the switching from the normal spectral efficienton-frequency operation to the mode where reception and transmission fromthe relay occur on orthogonal resources enables increased receiversensitivity for the repeater as the feedback signal occur on anothernon-interfering resource.

In a preferred embodiment, the mode switching unit is arranged to switchbetween a first mode wherein the transmit antenna side utilizes the sameresource as the receive antenna side and a second mode wherein thetransmit antenna side utilizes a different resource from the receiveantenna side. The use of different resources will reduce theself-interference from the transmitting side to the receiving side.Preferably, in the second mode, the transmit antenna side utilizes aresource that is substantially orthogonal to the resource used by thereceive antenna side, as this will minimize the self-interference.

In another preferred embodiment, the mode switching unit is arranged toswitch between a first mode wherein the transmit antenna side utilizes adifferent resource from the receive antenna side and a second modewherein the transmit antenna side utilizes a different resource from thereceive antenna side, said first and second mode enabling different waysof changing the resource. This will enable a switch between, forexample, a first mode in which translation is made between a first and asecond frequency and a second mode in which translation is made betweena first and a second code.

Preferably, at least a first threshold is defined and the mode switchingunit is arranged to switch to the first mode when the requiredamplification gain is below the first threshold. The mode switching unitmay be further arranged to switch to the second mode when the requiredamplification gain is above the first threshold.

As an added feature, the mode switching unit may be arranged to switchto a third mode when the required amplification gain is above a secondthreshold which is higher than the first threshold. This will enable apredefined structure of different translation modes, depending on whichmode provides the best protection against self-interference.

In a preferred embodiment the node comprises a mode selector unit forselecting a mode and for feeding information about the selected mode tothe mode switching unit. Alternatively, the step of selecting the modemay be performed in a sending node and/or a receiving node andinformation about which mode to select may be communicated from thesending and/or receiving node to the repeater node. In the latter case,the mode switching unit may be arranged to receive from a sending nodeand/or a receiving node a message about which mode to select. Theinformation about which mode to select may be in the used directly asinput to the mode selector unit, or may be used as a recommendation bythe node when selecting the mode.

The invention also relates to a wireless communications systemcomprising at least one base station arranged to communicate with atleast one mobile terminal and at least one repeater node or relay nodecomprising a receive antenna for receiving a signal through a wirelessconnection, an amplifier for amplifying the signal and a transmitantenna for forwarding the amplified signal, characterized in that saidrepeater node or relay node is a node as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following, by wayof example and with reference to the appended drawings in which:

FIG. 1 illustrates an on-frequency repeater function

FIG. 2 illustrates a repeater function utilizing different resources forreceiving and transmitting

FIGS. 3 a-3 c illustrates methods of using different resources.

FIG. 4 illustrates a relay or repeater node according to the invention

FIG. 5 is a flow chart of a method according to the invention.

FIG. 6 illustrates the control message resulting from the mode switchingfunction executed at least partially at the sender, the receiver or bothnodes with which the repeater/relay node is in communication.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates an on-frequency repeater function, that is, therepeater receives and transmits on the same resource at substantiallythe same time, hence causing self interference from the output to theinput, although in practice there will always be a small delay. Therepeater 1 comprises a receive antenna 3 arranged to receive a signalfrom a base station 5 and a transmit antenna 7 arranged to forward thesignal to a mobile terminal 9. An amplifier 10 provides a gain betweenthe receive antenna 3 and the transmit antenna 7. The signal isforwarded on the same resource on which it was received. This means thatthe repeater 1 uses the same frequency, non-orthogonal coding etc. andforwards the signal immediately upon receiving it.

Such repeaters are prone to self-interference, and are thereforetypically designed so that the transmit and receive antennas areisolated from each other to the extent possible. As discussed aboveself-interference cancellation methods based on feedback signals betweenthe transmit antenna and the receive antenna are used to reduceself-interference. In FIG. 1 this feedback signal is illustrated by aunit W indicating that it applies a (complex valued) weighting of thefeedback signal. Still, if the signal power feed back to the inputbecomes too high, self-interference cannot be completely ruled out insuch repeaters. Self-interference is illustrated in FIG. 1 by a dashedline from the transmit antenna to the receive antenna.

FIG. 2 illustrates a repeater 21 arranged to forward the received signalon a different resource from the one on which it was received.Preferably, the resources are substantially orthogonal, to minimize theinterference between them. For example, the repeater 21 may be afrequency translating repeater. Again the repeater 21 comprises areceive antenna 23 arranged to receive a signal from a base station 25and a transmit antenna 27 arranged to forward the signal to a mobileterminal 29. To illustrate the use of different resources, the receivingfunction and the transmission function are shown in two different partsof the figure, a) and b), respectively.

FIGS. 3 a-3 c illustrate different ways to utilize different resources.In each of these figures the horizontal axis represents time t. In FIG.3 a, the receive function Rx and the transmit function Tx are assigneddifferent time intervals, so that the repeater 11 never transmits andreceives at the same time. In FIG. 3 b the receive function Rx and thetransmit function Tx are assigned different frequency domains, f1 andf2, respectively. In FIG. 3 c the receive function Rx and the transmitfunction Tx are assigned different orthogonal spreading codes, code1 andcode2, respectively.

FIG. 4 illustrates a repeater 21 according to an embodiment of theinvention, which is able to switch between a first and a second mode ofoperation depending on the conditions. As before, the repeater 21comprises a receive antenna side having at least one receive antenna 23arranged to receive a signal from a base station 25 and a transmitantenna side having at least one transmit antenna 27 arranged to forwardthe signal to a mobile terminal 29. An amplifier 30 provides a gainbetween the receive antenna 23 and the transmit antenna 27. The repeater21 also comprises a mode switch 31 arranged between the receive antenna23 and the transmit antenna 27 in series with the amplifier 30. The modeswitch is arranged to switch between a first and a second mode ofoperation of the repeater 21. Which mode to switch to may be determinedby a mode selector 33 arranged in the repeater 21. Alternatively, it maybe determined in the sender or the receiving node, or possibly in adistributed fashion over those nodes. In the latter case, informationabout the selected mode can be received in the repeater node 21 by meansof the receive antenna 23 and processed in the mode switch 31. Therepeater 21 may also comprise other units, for example, a feedback unit,as shown in FIG. 1 for interference cancellation.

In the first mode the relay or repeater will transmit concurrently whilereceiving on the same frequency. This is suitable in situations wherethe SIR is good, which will often be the case, for example, when thepath loss is low. In the second mode, the relay/repeater node will usedifferent resources for receiving and transmitting. The resources arepreferably orthogonal, to minimize interference between them. Asdiscussed in connection with FIGS. 3 a-3 c, the resources may be dividedin time, in frequency and/or in the code domain.

One advantage of the time duplex operation shown in FIG. 3 a is that therelay/repeater node can decode the received signal and retrieve thepayload information without noise. The noise free version can then beamplified and transmitted instead of amplifying a signal comprising boththe payload and the noise.

In a preferred embodiment, the condition for selecting the first or thesecond mode is whether the required relay/repeater node gain is above orbelow a gain threshold value G₀. The gain threshold value willinherently be determined by the relay/repeater node design, i.e. theisolation between the transmit and receive antennas, whether anyinterference cancellation is applied, etc. First, the gain needed in therelay/repeater node can be determined as

G _(RN) =P _(RN) ^((TX)) /P _(RN) ^((RX)) =P _(MN) ^((RX)) G _(RN→MN) /P_(RN) ^((RX)).   (1)

where

TX and RX stand for transmit and receive respectively,

RN denotes for relay/repeater node,

MN denotes the mobile node (or rather the receiving node), and

P indicates power and

G indicates gain.

The repeater amplification gain is based on at least the repeater to thereceiver(s) channel gain, where the considered receiver or receivers maychange over times, and when a group of receivers are targeted, thechannel gain to the receiver with the 0 worst performance is considered.

As can be seen from Eq. 1, assuming that the desired MN receive-powerP_(MN) ^((RX)) is known, and the relay/repeater node receive-powerP_(RN) ^((RX)) is given, the required relay node gain depends (mainly)on the relay/repeater node to mobile node channel gain G_(RN→MN). Thedesired MN receive-power P_(MN) ^((RX)) and the relay/repeater nodereceive-power P_(RN) ^((RX)) are both given by the quality of servicerequirements, bit error rate, data rate, and noise plus interferencesituation.

The relay node gain G_(RN) should be compared to the threshold. Theswitching between the modes in this embodiment can be described through

if G_(RN)<G₀, then Mode 1

else if G_(RN)≧G₀, then Mode 2   (2)

As mentioned above, in the preferred embodiment the second mode canutilize different resources in terms of time, frequency or coding. In analternative embodiment, both the first and the second mode involve usingdifferent resources for transmitting and receiving. In this case, thefirst mode may be, for example, using different codes for transmittingand receiving, while the second mode may be using different frequencies,or separating the received and transmitted signals in time.

More than one threshold may be used, to enable selection between severaldifferent modes. In this case, the first mode may still be using thesame resource for transmitting and receiving and two or three additionalmodes may be defined in which receiving and transmitting use differentresources. Alternatively all modes may use different resources. Forexample, the first mode may be using different orthogonal spreadingcodes, the second mode using different frequencies and the third modeseparating the signals in time. The selection between several differentmodes may be performed in one step, or in several steps. In the formercase, each mode is assigned to an interval of amplifier gain values andthe mode matching the actual amplifier gain is selected. In the lattercase, assuming a first mode is used, a switch to the second mode isperformed when the amplifier gain passes a first threshold and a switchto the third mode is performed when the amplifier gain passes a secondthreshold.

In order to determine which mode to operate in, by calculating the gainor the relay/repeater node, at least the gain between the relay/repeaternode to the receiving node must be determined. This can be done inseveral different ways. For example the link gain may be measuredproactively and reported to the mode selector, which, as stated abovemay be located in the sender, the receiver or the relay/repeater node,or may be distributed between them. Alternatively, a reactive type ofoperation may be used.

In one possible reactive embodiment of the invention, a connection isalways initially set up in the second mode, that is, in the mode wherethe relay/repeater receives and sends on different resources. In thisstarting phase, the channel and gains are measured and if suitable, therelay/repeater node is switched to operate in the first mode.

FIG. 5 is an example of a reactive embodiment of the method as discussedabove.

In step S51 a default mode of operation is applied when amplifying andforwarding signals. This default mode is preferably the second mode.

In step S52 a required amplifying gain is determined.

In step S53 the required amplifying gain is compared to a predeterminedthreshold.

In step S54 a mode of operation is selected in dependence of the resultof the comparison performed in step S53. As discussed above, theselection may be performed in the repeater node itself, in the sender ortransmitter, or distributed between two or more of these units.

In step S55 the selected mode of operation is applied to the repeaternode.

In a proactive embodiment the procedure may instead start with measuringthe link gain and using this to determine the required amplifying gainin step S52.

As will be understood, different relaying modes may be usedsimultaneously in the uplink and downlink communication through the samerelay or repeater node.

FIG. 6 illustrates a situation similar to that of FIG. 2, in which thebase station 25 and/or the mobile terminal 29 comprise logic units 35,37, respectively, arranged to determine the preferred mode of operationto be used by the relay node 21. As illustrated by the dashed arrows inFIG. 6, the base station 25, the mobile terminal 29, or both, may send amessage, conveying information, to the repeater/relay node 21 regardingoperation mode. The message may be an order that has to be followed or arecommendation that the relay/repeater node 21 based on furtherinformation housed or collated by the repeater/relay that account forwhen deciding which mode to operate in. In the latter case, the node 21may base its decision on information received from either the basestation 25, the mobile terminal 29, or both.

While the description above has been based on downlink communication,that is, communication from the base station 25 to the mobile terminal29, the skilled person will readily understand that the structure andfunction of the inventive repeater node will be completely analogous tothat described above regardless of the nature of the sending andtransmitting node. That is, the teachings of this document could beapplied to uplink or downlink communication, as well as to communicationbetween any two units involving the use of a repeater and/or relay nodebetween the units. Therefore, the scope of the claims should not belimited to communication in any particular direction, or between anyparticular type of communicating units.

1. A node for use in a wireless communications system, said nodecomprising: a receive antenna side having at least one receive antennafor receiving a signal through a wireless connection; an amplifier foramplifying the signal and a transmit antenna side having at least onetransmit antenna for forwarding the amplified signal; and a modeswitching unit for switching between at least a first and a second modeof operation of the node in dependence of an amplification gainrequirement.
 2. A node according to claim 1 wherein the mode switchingunit is arranged to switch between a first mode wherein the transmitantenna side utilizes the same resource as the receive antenna side anda second mode wherein the transmit antenna side utilizes a differentresource from the receive antenna side.
 3. A node according to claim 2,wherein the transmit antenna side in the second mode utilizes a resourcethat is substantially orthogonal to the resource used by the receiveantenna side.
 4. A node according to claim 1, wherein the mode switchingunit is arranged to switch between a first mode wherein the transmitantenna side utilizes a different resource from the receive antenna sideand a second mode wherein the transmit antenna side utilizes a differentresource from the receive antenna side said first and second modeenabling different ways of changing the resource.
 5. A node according toclaim 1, wherein the mode switching unit is arranged to switch to thefirst mode when the required amplification gain is below a firstthreshold.
 6. A node according to claim 5, wherein the mode switchingunit is arranged to switch to the second mode when the requiredamplification gain is above the first threshold.
 7. A node according toclaim 5, wherein the mode switching unit is arranged to switch to athird mode when the required amplification gain is above a secondthreshold which is higher than the first threshold.
 8. A node accordingto any claim 1, further comprising a mode selector unit for selecting amode and for feeding information about the selected mode to the modeswitching unit.
 9. A node according to claim 1, wherein the modeswitching unit is arranged to receive from a sending node and/or areceiving node a message about a preferred mode.
 10. A method ofrepeating a signal in a cellular network, said method comprising:receiving the signal in a node comprising a receive antenna side havingat least one receive antenna, a transmit antenna side having at leastone transmit antenna, and an amplifier between said receive antenna sideand said transmit antenna side, amplifying the signal; transmitting theamplified signal; and selecting one of a first mode and a second mode ofoperation for the node in dependence of an amplification gainrequirement.
 11. A method according to claim 10, wherein selecting themode comprises selecting between a first mode wherein the signal isreceived and transmitted using the same resource and a second modewherein the signal is transmitted on a different resource from that onwhich it was received.
 12. A method according to claim 10, wherein thesecond mode involves transmitting the amplified signal on a resourcethat is substantially orthogonal to the resource used by the receiveantenna side.
 13. A method according to claim 10, wherein selecting themode comprises switching between a first mode wherein the transmitantenna side utilizes a different resource from the receive antenna sideand a second mode wherein the transmit antenna side utilizes a differentresource from the receive antenna side, said first and second modeenabling different ways of varying the resource.
 14. A method accordingto claim 10, wherein the first mode is selected when the requiredamplification gain is below a first threshold.
 15. A method according toclaim 14, wherein the second mode is selected when the requiredamplification gain is above the first threshold.
 16. A method accordingto claim 14, further comprising selecting a third mode when the requiredamplification gain is above a second threshold which is higher than thefirst threshold.
 17. A method according to claim 10, wherein selectingthe mode is performed in the node.
 18. A method according to claim 10,wherein selecting the mode comprises identifying a preferred mode in asending node and/or a receiving node and communicating information aboutthe preferred mode from the sending and/or receiving node to therepeater node.
 19. A wireless communications system comprising: at leastone base station arranged to communicate with at least one mobileterminal; and at least one repeater node or relay node, said relay nodecomprising a receive antenna for receiving a signal through a wirelessconnection, an amplifier for amplifying the signal and a transmitantenna for forwarding the amplified signal, said repeater node or relaynode configured to perform the method of claim 1.